The field of the invention is positron emission tomography (PET) scanners, and particularly the filtering of acquired transmission data used to reconstruct an image.
Positrons are positively charged electrons which are emitted by radionuclides that have been prepared using a cyclotron or other device. These are employed as radioactive tracers called "radiopharmaceuticals" by incorporating them into substances, such as glucose or carbon dioxide. The radiopharmaceuticals are injected in the patient and become involved in such processes as blood flow, fatty acid, glucose metabolism, and protein synthesis.
As the radionuclides decay, they emit positrons. The positrons travel a very short distance before they encounter an electron, and when this occurs, they are annihilated and converted into two photons, or gamma rays. This annihilation event is characterized by two features which are pertinent to PET scanners--each gamma ray has an energy of 511 keV and the two gamma rays are directed in nearly opposite directions. An image is created by determining the number of such annihilation events at each location within the field of view.
The PET scanner includes one or more rings of detectors which encircle the patient and which convert the energy of each 511 keV photon into a flash of light that is sensed by a photomultiplier tube (PMT). Coincidence detection circuits connect to the detectors and record only those photons which are detected simultaneously by two detectors located on opposite sides of the patient. The number of such simultaneous events indicates the number of positron annihilations that occurred along a line joining the two opposing detectors. Within a few minutes hundreds of millions of events are recorded to indicate the number of annihilations along lines joining pairs of detectors in the ring. These numbers are sorted into an array known in the art as a "sinogram", in which each row records the events at a particular view angle (.theta.) and each column records events at a particular distance (R) from the isocenter. These numbers are corrected for system errors and employed to reconstruct an image using well known computed tomography techniques.
Filtering of transmission data along the sinogram row direction (R) is often employed to improve transmission statistics, and to thereby reduce the variance of the transmission data and improve the quality of the reconstructed emission image. However, this filtering leads to systematic errors in the attenuation correction and it is common practice to provide the operator with control of a filter parameter so that he may make the tradeoff between noise reduction in the image and the introduction of systematic errors. The choice will be determined by imaging circumstances and a filter parameter, such as filter width in millimeters, is adjusted by the operator to optimize the image. The effect of this filtering in the row direction (R) is uniform throughout the field of view and is, therefore, relatively easy to predict.
Transmission statistics can also be improved by filtering the transmission data in the sinogram in the column direction (.theta.). However, the resolution impact of filtering in the .theta. direction is less at the center of the field of view than at the edges, and this makes the selection of a filter in any given situation more complex. As a result, PET scanners typically do not enable the operator to select filter parameters for .differential. direction filtering because the impact on the resulting image is too complex.